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Poisoned Water | Protecting Pipes from the Effects of Corrosion

Learn about the chemistry involved in water pipes and why a failure to protect against corrosion in the water system of Flint, Michigan, tainted city water with lead and a dangerous strain of bacteria, in this media gallery from NOVA: Poisoned Water. Mineral “scale” that builds up inside water pipes serves a protective function: it prevents metal in the pipes from dissolving in water. For the scale to do its job, it must be protected from a corrosive chemical environment. However, when an anti-corrosive chemical has not been used, corrosive water flowing through a metal pipe can cause the scale to deteriorate. This can prompt reactions that leach lead from the pipe into the water and also allow disease-carrying germs to spread. This resource is part of the NOVA Collection.

Natural factors combine with human inputs to give each source of drinking water—a river, pond, lake, or aquifer—a unique chemical signature. The Flint water crisis underscores the importance of treating water according to the source’s signature. The Michigan city’s failure to do so produced grave consequences.

A network of pipes carries public drinking water from a treatment plant to residential and nonresidential customers. As water flows, certain minerals present in the water gradually precipitate out of solution and help form a hard coating that lines the inside of the metal pipes. This lining, called scale, protects the metal from direct contact with the water, which otherwise could cause corrosion. Corrosion occurs when metal is exposed to dissolved oxygen or chloride ions in water. The chemical reaction releases metal into the water in soluble form. For iron pipes, which are commonly used as a public water system’s main distribution pipes (the “mains”), corrosion produces rust, turning water brown or orange. While rust may also affect water’s smell and taste, rusty water is not toxic or dangerous to drink.

Lead, on the other hand, is a serious toxin. It is invisible, odorless, and tasteless in soluble form. Despite being banned from use in plumbing materials in 1986, lead can still be found in many smaller lines that branch from the mains and service homes and businesses. Lead is also a component of solder, which connects pipe sections together. When water sits in corroded lead pipes, lead can leach into the water supply. To control pipe corrosion and guard against these outcomes, treatment plants typically introduce chemical additives to treated water. These so-called corrosion inhibitors help build up the protective scale inside the pipes without harming water quality.

In 2014, the city of Flint decided to change its water source from Lake Huron to the Flint River. Prior to the switch, Lake Huron water, which was treated and delivered by the Detroit Water and Sewerage Department, had flowed through Flint’s water system for decades. The scale that formed inside the distribution pipes was a product of that water plus a corrosion inhibitor that Detroit Water and Sewerage added as part of its treatment regimen. Over time, this had created a stable system inside the pipes. Switching the water supply to Flint River water changed the chemistry.

After the switch, the city would use its own water treatment facility to treat Flint River water for distribution to its customers. The Flint River contains about nine times more chloride ions than water from Lake Huron. This is due in part to the influx of runoff, which contains road salt used to treat streets adjacent to the river in winter. Because of its high chloride content, Flint River water is more corrosive. As the system responded to the different water chemistry and sought to restore equilibrium, existing scale flaked off, exposing bare metal to the water and releasing toxic lead into the drinking water.

Corrosion produced two serious health problems in Flint, becoming evident within just months of the switch. Flint’s mains are mostly iron. However, many of its service lines are made of lead. Corrosion of these pipes and the lead-based solder that connected them caused perhaps the more serious health problem: lead poisoning. Symptoms of lead poisoning may include developmental delays and learning difficulties in children and memory loss, mood disorders, stomach cramps, and joint pain in all populations. The second health problem was related to a human pathogen: Legionella bacteria. Chlorine, which is typically added to disinfect a water supply and eliminate waterborne pathogens, readily reacts in the presence of iron. As Flint’s corroding mains leached iron into the water, the chlorine molecules reacted with the iron and were unable to prevent the Legionella from spreading. This prompted an outbreak of Legionnaire’s disease.

Here are suggested ways to engage students with the phenomenon highlighted in this video:

Before watching the video

Ask students to record the sequence of events and consequences that resulted from withholding corrosion-control chemicals.

During the video

Discuss the importance of scale in the lead pipes. Ask students to:

Identify the properties of the scale that help make the pipes safe for carrying drinking water.

Identify examples of everyday materials or recognizable structures that have a coating created by the oxidation of the original metal (e.g., rust on iron, the greenish patina of the Statue of Liberty).

Compare these coatings with scale in pipes. Which ones are beneficial?

Use this case, or a similar case such as Cincinnati or Washington, D.C., as an application context for student assessment of chemical equilibrium and the effects of changing the concentration of reactants in a chemical system.

Here are suggested ways you may use this video to support students’ engagement with science practices:

Ask students to refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level.

Ask students a scientific question about the effect of water chemistry or water systems that could be researched or tested.

Ask students to develop a conceptual or graphical model that can be used to explain the impact of changing amounts of chlorine or corrosion control in water systems.

Ask students to make an evidence-based claim, using evidence from the video or other resources, about how Le Chatelier’s principle applies to the Flint water crisis.

Have students synthesize information from research on other systems that rely on chemical equilibrium to function properly. (Examples can include swimming pools, fish tanks, vegetable gardens or crop fields, gas bubbles in soda drinks, or hemoglobin in blood.)

Here are suggested discussion questions about the phenomenon for students to consider:

What were the significant health consequences of withholding corrosion control from Flint’s water supply?

Describe the changes in properties of the materials in the water pipes that indicate a chemical reaction has occurred, both when corrosion control was present and when it was not. Include a discussion of both solubility and reactivity of the materials.

Describe the purpose and impact of chlorine in the water before and after the decision to withhold corrosion-control chemicals.

Many communities have lead water supply pipes that connect the municipal water system to individual houses. Describe the importance of oversight and informed decisions in those communities to keep the water systems safe.

The water distribution infrastructure in our country is very old. Describe the importance of corrosion control to maintain the integrity of the water infrastructure, safeguard water quality, and protect public health.